Docker Support

Motivation:

We're currently sync'ing some very large openshift repos from brew. In reality we're only interested in the current 3.9 and newerreleases/tags but we're getting everything, which is weighing in at 1.2TB at present and growing with each puddle for each version.

It would be nice to be able to limit the tags we sync.

Solution:

Provide a config on the DockerImporter config called "tags". This config will accept a list of tags to sync.

Provide a CLI option called "--tags". The user will be able to specify "--tags" when creating a docker repository. The user will also be able to override the "--tags" importer config at sync time, by specifying it as part of the override_configs.

When the "tags" option is provided:

Pulp will sync just those tags + manifests associated with the tags and all the corresponding layers. Sync will only be performed using the immediate download policy. There will be no support for the on_demand download policy.

Note: If the list of tags was changed, and tag X is not wanted anymore, a manual removal of that tag X should occur in the local repo.
Note: If some invalid tags provided are invalid, Pulp will only sync the available ones in the remote repo.

When the "tags" option is not provided:
Pulp will sync the whole repo.